It's easy to take antibiotics for granted. Stuart Levy knows that. He runs one of the most respected laboratories that study them, at Tufts University in Boston. He says we tend to forget that scientists developed the drugs barely 70 years ago.

If you feed antibiotics to pigs and chickens and cattle, the animals grow faster. Scientists still don't really know why, although they have theories.

"What makes an antibiotic a miracle is first that this group of drugs has reversed deaths from pneumonias, septic events in surgery, cured urinary tract infections, skin infections, diseases of the brainany tissue of the body. There has been antibiotics that could literally, in their heyday, have cured most, if not all, infections," Levy explains.

And back in the 1940s, farmers started using them to cure sick animals. Then researchers accidentally discovered that the drugs can work other miracles, too. If you feed them to pigs and chickens and cattle, they make the animals grow faster. Scientists still don't really know why, although they have theories. In any case, America's drug and meat companies have convinced farmers that you have to feed antibiotics to your animals almost every day if you're going to compete in this age of factory farming.